Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.678220
Title: Bio-tribology of total disc replacements of the lumbar spine
Author: Hyde, Philip James
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2012
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Abstract:
Total disc replacements (TDR) of the lumbar spine offer a direct replacement to the natural intervertebral disc (IVD). Possible indications for replacement would typically include severe discogenic pain that has not responded to conservative treatment. Until relatively recently the preferred surgical intervention was removal of the pain-causing disc and creation of a bony mass between the adjacent vertebrae - known as a fusion. Concerns over removing motion at one level and the effect of compensated motion at other levels was a driver in the development of motion preservation surgery using TDR. Most popular designs rely on technology directly translated from total hip replacement (THR) and total knee replacement (TKR). However, the design rationales used vary considerably and the operating regime in the spine is different to other joints which work by articulation in synovial fluid. Furthermore, the successes of the original generation of THRs were countered by late failures due to loosening (osteolysis); a result of adverse tissue reactions to particulate wear debris. Therefore pre-clinical simulation using a stratified approach (including parametric studies) should be a goal in development of both better understanding of present TDR performance and in aiding in design of the next generation devices and avoiding sub-optimal design. The aim of this thesis was to exam me performance of two different devices, representing opposing design rationales, and use parametric studies based on the international testing standard (ISO 18192-1) to assess performance characteristics. The TDRs were challenged by altering the phasing, loading and amplitude of the cyclical input motions. Further to this the effects of short strokes encountered in spinal motions were investigated. To compliment these results, experiments investigating contact pressure, motion paths, friction and lubrication were completed. The resultant rates of wear (in mg/million cycles (mgIMC) ± standard deviation) of Prodisc and Charite TDR devices were comparable, demonstrating wear rates of 16.1 ± 1.4 mgIMC and 13.4 ± 2.0 mglMC respectively for the standard ISO study. The following ISO-based experiments highlighted the impact that small perturbations of the testing regime can have. Surface profilometry and secondary electron microscopy demonstrated a wear phenomenon not observed in vivo. where micron-sized particles of ultra-high molecular weight polyethylene (UHMWPE) appeared to be re-attached to the surface. Edge-effects on the UHMWPE surfaces adjacent to the radii of curvature of the fillets of the metallic endplates produced burnishing and deformation which were quantitatively recorded in terms of roughness parameters. Motion tracking of TOR components highlighted the similarity between ISO motions and in vivo based kinematics from Callaghan et at. [1]. Frictional characteristics were studied and found to be in the range 0.05-0.09 which is similar to those reported in MoP hips. Lubrication was theoretically studied and found to be boundary in nature for contemporary TOR designs. Neither TOR design excelled over the other when tested under the parametric studies described here. Rates of wear, though lower than historic THR and TKR wear rates, were high enough to raise the concern about possible future osteolysis development. Even small amounts of cross shear (ratio 0.03) were enough to cause significant mass loss and hence debris production. Reduced loading did not reduce the wear substantially and may indicate that patient weight will not affect TOR performance in terms of rates of wear alone. Surface topography was monitored for the two devices and input kinetics. This highlighted the correlation between axial loading and roughening of the pole area of the PE components; caused by particles of re-attached debris at the bearing surface. Edge-effects were measured in terms of surface features around the PE bearing perimeter and related to the higher than expected contact stress in this region. Frictional torques were investigated and found to be in the region of 1.5 Nm. This is not of concern per se, however, the artificial TOR and the natural IVD differ in two key respects: I) in the articulating TDR the resistance to rotation is reasonably constant; 2) is not proportional to angle of flexion. The in vivo effects of this difference are as yet unknown. In conclusion, the bio-tribological operating regime is harsh and therefore future bearing design should consider how to be more resilient to this at the design stage.
Supervisor: Hall, Richard ; Fisher, John Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.678220  DOI: Not available
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